(Mg,Mn,Fe,Co,Ni)O: A rocksalt high-entropy oxide containing divalent Mn and Fe

High-entropy oxides (HEOs) have aroused growing interest due to fundamental questions relating to their structure formation, phase stability, and the interplay between configurational disorder and physical and chemical properties. Introducing Fe(II) and Mn(II) into a rocksalt HEO is considered challenging, as theoretical analysis suggests that they are unstable in this structure under ambient conditions. Here, we develop a bottom-up method for synthesizing Mn- and Fe-containing rocksalt HEO (FeO-HEO). We present a comprehensive investigation of its crystal structure and the random cation-site occupancy. We show the improved structural robustness of this FeO-HEO and verify the viability of an oxygen sublattice as a buffer layer. Compositional analysis reveals the valence and spin state of the iron species. We further report the antiferromagnetic order of this FeO-HEO below the transition temperature ~218 K and predict the conditions of phase stability of Mn- and Fe-containing HEOs. Our results provide fresh insights into the design and property tailoring of emerging classes of HEOs.


Supplementary Text Experimental details Synthesis of oxalate precursor
The synthesis of oxalate precursor involves a co-precipitation of different metal ions into fine particles.Some involved divalent cations (i.e., Fe 2+ and Mn 2+ ) are vulnerable to oxygen, meaning that they are prone to be oxidized to their higher valence states.Therefore, the fabrication of the precursor was undertaken in an inert atmosphere using Schlenk lines, guaranteeing a pure Ar atmosphere during the wet synthesis.In a typical procedure, 0.1 mmol ascorbic acid (C6H8O6) was first dissolved in a mixed solution of 15 ml deionized H2O and 15 ml ethylene glycol.Ascorbic acid, which is a reducing agent, can effectively prevent the oxidation of Fe 2+ and Mn 2+ in the aqueous solution.Then, 1.1 mmol MgCl2 (98%), 1 mmol MnCl2•4H2O (98%), 1 mmol FeCl2 (98%), 1 mmol CoCl2•6H2O (99%), and 1 mmol NiCl2•6H2O (100%) were dissolved into the above solution in a round bottom flask.After all metal chlorides were added to the solution, the flask was swiftly connected to the Schlenk line and purged with argon gas for three times.This solution of metal chlorides was warmed up to 50 °C under stirring.In another mixed solution of deionized H2O and ethylene glycol (15 ml+15 ml), 5.1 mmol ammonium oxalate monohydrate (NH4)2C2O4•H2O was slowly dissolved at 50 °C.This solution was then deoxygenated using the Schlenk line before being injected into the solution of chlorides under vigorous stirring.After a reaction for 6 hours, the oxalate precursor was washed and separated via centrifugation for a couple of times, followed by drying the precursor at 50 °C overnight.

Annealing of oxalate precursor
A single-phase FeO-HEO can be obtained by annealing the precursor at a high temperature.To avoid the oxidation of Fe 2+ and Mn 2+ , we conducted the annealing in a tube furnace filled with argon.The tube atmosphere was purged with pure Ar for three times before the annealing.A certain amount of oxygen was introduced to the system to effectively counteract the reductive environment created by the thermal decomposition of oxalate in inert atmosphere.Specifically, the precursor was calcined using a tube furnace at 1000 °C for 6 h with a ramp rate of 10 °C/min in an Ar atmosphere within a partially lidded corundum ceramic boat.A measured 300 mg of precursor was placed in a quartz pan within the boat, and 90 mg of MnO2 as an oxygen generator was situated next to the quartz pan.The use of quartz pan allowed the MnO2 to be situated close to the precursor sample, while preventing contact and contamination.The FeO-HEO can be obtained after cooling down the sample naturally to the room temperature, and the as-obtained dark brown powders are proved to be stable in ambient environment, as shown in Figure S5, the phase purity shows no change after exposing the sample to air for months.

Characterizations
The chemical compositions of the oxalate precursor were analyzed by using inductively coupled plasma optical emission spectroscopy (ICP-OES).10 mg of oxalate sample was dissolved in 20 ml 0.1 M nitric acid at the room temperature.Analysis was undertaken using corresponding calibration solutions.
All ex-situ X-ray diffraction patterns were acquired on a Rigaku Ultima IV laboratory diffractometer with Cu Kα radiation in plate mode.The in-situ XRD measurements were performed on the diffractometer with a high-temperature attachment.The sample was heated up to different stages with a ramp rate of 10 °C/min, and a dwelling time of 10 min was adopted before each scan.
Quasi in-situ XPS measurements were carried out on Thermo Escalab 250 XI with a monochromatic Al-Kα source.The sample was annealed at different temperatures in an isolated oven and was transferred into the ultra-high vacuum chamber without exposure to air.All spectra were initially calibrated with the C 1s peak of adventitious carbon at 284.8 eV before any fitting.A Mössbauer spectrum of 44 mg/cm 2 of (Mg,Mn,Fe,Co,Ni)O powder was recorded at room temperature in the ± 4 mm/s velocity range with a krypton gas proportional counter recording the 1.8 keV Kr K-α escape peak and 14.41 keV resonant photons using a Wissel CMCA-500 multichannel analyzer.The spectrometer was calibrated at room temperature using α-iron, which serves as isomer shift reference.
Magnetic properties were measured on a piece of pressed (MgMnFeCoNi)O pellet with a Quantum Design (QD) Magnetic Property Measurement System in the temperature range 2.0 < T/K <350 K and in applied magnetic fields 1 T and at 2 K in variable field between 0 and 6 T. Zero-field cooled and field cooled data were collected between 2 and 320 K under with an applied field of 0.1 T. The specific heat data were collected on a 16.6 mg pellet of (MgMnFeCoNi)O between 2 and 350 K using a 9T QD Physical Property Measurement System (PPMS) in zero applied magnetic field.SEM images were taken on a field-emission scanning electron microscope (ZEISS Gemini 500) operating at 30 kV.The atomic-scale characterizations of individual FeO-HEO particle were conducted on an aberration-corrected STEM (FEI Titan Cubed Themis G2 300, FEI) at an accelerating voltage of 300 kV with a convergence semi-angle of 25 mrad.

Wet-chemistry synthesis of oxalate precursor
The synthesis strategy of a precursor using oxalate anion as the 'bridging' ligand will be elucidated specifically in another work published elsewhere.Critical stability constants of involved metaloxalate complexes are listed in Table S1.The complexes formed between oxalate and Ni 2+ are the most stable among these five cations, followed by the Co 2+ containing complexes.The stabilities of Fe-and Mg-oxalate complexes are relatively close and slightly lower than others.The chemical compositions determined by ICP-OES are shown as the inset in Figure S1A.The molar fraction of each metal component is close to 20%, while that of Mg is lower than others reaching a percentage of 18%.This diminution in Mg content may be ascribed to the slight dissolution of Mg oxalate during washing.Figure S1 show the SEM images of oxalate precursor.The images at low magnification (Figure S1B) and high magnification (Figure S1C) indicate that the as-synthesized oval precursor is about 3 to 4 μm in length and has a maximum width of 800 nm around the waist.

Annealing of oxalate precursor
As shown in Figure S2, direct annealing of the oxalate precursor in air results in the formation of a spinel product, which is due to the further oxidization of Co 2+ and Fe 2+ to their higher valence states.Therefore, to synthesize the single-phase rocksalt FeO-HEO, in which different metal ions cooccupy the cation sites, all cations are supposed to be bivalent in this structure.The precursor was then annealed in an argon atmosphere with a certain amount of oxygen generator (that is, MnO2 in our case) to obtain a rocksalt FeO-HEO.
However, the annealing process in pure Ar leads to a mixture of wüstite (FeO) and Ni alloy (Figure S3A).The presence of metallic phase can be ascribed to the generation of reductive by-products (i.e., carbon and carbon monoxide) during the heat treatment (42).In this regard, MnO2 is introduced into the annealing process as an oxygen generator to offset the reductive environment.MnO2 decomposes progressively at high temperatures and releases a small amount of O2, either neutralizing the reductive substances or slightly re-oxidizing the as-formed metallic products.MnO2 undergoes a thermal decomposition as follows (43): (Above 800°C) Figures S3 B-D show the XRD patterns of the samples annealed with the addition of MnO2 as an external oxygen source.An excess of MnO2 produces a spinel product (Figure S3B).This spinel phase forms because Fe 2+ , Mn 2+ , and Co 2+ cations can be possibly oxidized to Fe 3+ , Mn 3+ , and Co 3+ by extra oxygen generated from MnO2.A variety of normal and inverse spinel oxides are likely to form during this process.(82) Hence, it is necessary to control the amount of MnO2 properly.
Figure S3C exhibits the co-existence of spinel and metallic phases in the annealed product with the controlled addition of MnO2. Figure S4 illustrates an insufficient annealing process schematically.Specifically, due to a limited oxygen diffusion rate within the fixed bed, insufficient annealing results in an overoxidized top layer and an under oxidized bottom layer.Such a problem can be addressed by prolonging the annealing time.As shown in Figure S3D, compared with the product in Figure S3C, when the precursor was annealed at the same temperature with the same amount of MnO2, an annealing time more than 5 hours results in well-crystallized single-phase FeO-HEO.S7.These refinement results suggest a centrosymmetric fcc lattice with a unit-cell parameter a=b=c=4.282Å, which agrees with the results obtained from XRD refinement and NPD data collected at SNS, ORNL.An absence of overlap between the FC and ZFC curves indicates the presence of a small amount of ferromagnetic impurity.The similarity in MH curves suggests the retention of this weak ferromagnetism even at room temperature.(49).Scattering cross-section (σ) is commonly employed to describe the scattering power of X-rays and neutrons by a certain atom.This scattering cross-section is proportional to the square of scattering length (b), which can be expressed as σ = 4πb 2 .Therefore, neutron diffraction is a powerful complementary tool for XRD to characterize the crystal structures and distinguish elements in the proximity.

Fig. S1 .
Fig. S1.Characterizations of the oxalate precursor.ICP-OES results (A) and SEM images of oxalate precursor at low magnification (B) and high magnification (C).

Fig
Fig. S2.XRD pattern of the sample annealed at 1000 °C for 1 h in air.

Fig. S4 .
Fig. S4.Schematic illustration of an insufficiently annealed fixed bed in the tube furnace.

Fig. S6 .
Fig. S6.Calculated thermodynamic data of the reduction of involved binary oxides.Figure S6 shows the calculated thermodynamic data of the reduction of constituent binary oxides in a 5% H2 atmosphere.It is apparent that only NiO and CoO among all component oxides can be reduced into their metallic states (∆G < 0) in a temperature range from 0 °C to 1000 °C.

Fig. S8 .
Fig. S8.Fourier transforms of (k)k 3 oscillations (top scatter plots) for Mn, Fe, Co, and Ni K-edges and respective imaginary parts (bottom scatter plots).The curve fits of chi(R) data (solid) and imaginary parts (dash) are demonstrated as colored lines.The fit range is from 1 to 3 Å consistently in all cases, and fit windows are shaded in grey in each figure.

Fig. S10 .
Fig. S10.The decrease in the mixing enthalpies due to the change in the stability from Mn2O3 (Fe2O3) to MnO (FeO).

Table S5 . Mössbauer spectral parameters obtained from a fit to the room-temperature data.
Mössbauer spectrum with four components, where , , and ∆EQ, are the full width at half-maximum, the isomer shift, and the quadrupole splitting, respectively.Ca Co Cu Fe Mg 15 Ca Co Fe Mn Zn 29 Ca Cu Mg Ni Zn 43 Co Cu Mg Mn Zn 2 Ca Co Cu Fe Mn 16 Ca Co Fe Ni Zn 30 Ca Cu Mn Ni Zn 44 Co Cu Mg Ni Zn 3 Ca Co Cu Fe Ni 17 Ca Co Mg Mn Ni 31 Ca Fe Mg Mn Ni 45 Co Cu Mn Ni Zn 4 Ca Co Cu Fe Zn 18 Ca Co Mg Mn Zn 32 Ca Fe Mg Mn Zn 46 Co Fe Mg Mn Ni 5 Ca Co Cu Mg Mn 19 Ca Co Mg Ni Zn 33 Ca Fe Mg Ni Zn 47 Co Fe Mg Mn Zn 6 Ca Co Cu Mg Ni 20 Ca Co Mn Ni Zn 34 Ca Fe Mn Ni Zn 48 Co Fe Mg Ni Zn 7 Ca Co Cu Mg Zn 21 Ca Cu Fe Mg Mn 35 Ca Mg Mn Ni Zn 49 Co Fe Mn Ni Zn 8 Ca Co Cu Mn Ni 22 Ca Cu Fe Mg Ni 36 Co Cu Fe Mg Mn 50 Co Mg Mn Ni Zn 9 Ca Co Cu Mn Zn 23 Ca Cu Fe Mg Zn 37 Co Cu Fe Mg Ni 51 Cu Fe Mg Mn Ni 10 Ca Co Cu Ni Zn 24 Ca Cu Fe Mn Ni 38 Co Cu Fe Mg Zn 52 Cu Fe Mg Mn Zn 11 Ca Co Fe Mg Mn 25 Ca Cu Fe Mn Zn 39 Co Cu Fe Mn Ni 53 Cu Fe Mg Ni Zn 12 Ca Co Fe Mg Ni 26 Ca Cu Fe Ni Zn 40 Co Cu Fe Mn Zn 54 Cu Fe Mn Ni Zn 13 Ca Co Fe Mg Zn 27 Ca Cu Mg Mn Ni 41 Co Cu Fe Ni Zn 55 Cu Mg Mn Ni Zn 14 Ca Co Fe Mn Ni 28 Ca Cu Mg Mn Zn 42 Co Cu Mg Mn Ni 56 Fe Mg Mn Ni Zn